The present invention relates to a method and apparatus for continuous analysis of a gas and particulate stream. More particularly, the present invention provides, for example, for continuous analysis of an exhaust stream from Hall cells in the production of aluminum comprised of gaseous and particulate metal fluorides to determine fluoride concentration in such stream.
Conventional methods of sampling gas and particulate streams generally have required a manual or semiautomatic technique for collection of the particulate species of interest. An example of such a technique has involved collection of the particulate on a filter. The particulate so collected must be analyzed at a future time by any suitable analytical method. A disadvantage of such future analysis is that such analysis provides only time-averaged historical data. If process control or process performance information is required, a method and apparatus for continuously providing a reasonably accurate sample to a means for continuous analysis is needed for efficient process control.
A significant problem with most present systems for sampling particulate containing gas streams for continuous analysis is that the sample line becomes plugged with particulate. Such plugging of the sampling line may tend to bias any analysis by failure to collect all particulate species of interest.
U.S. Pat. No. 3,960,523 to Ryan discloses an apparatus and method for the continuous sampling of effluent-bearing gas for detecting contaminants therein. In operating Ryan's apparatus, the effluent-bearing gas enters a main chamber wherein it is contacted with a vaporized scrubbing liquid to remove the effluent. The gas is then discharged from the main chamber and the vaporized liquid is condensed and introduced to a reservoir. Preferably, a continuous flow of liquid from the reservoir is maintained to an analyzer for continuous analysis for gaseous borne effluent or contaminants. Ryan indicates that the method and apparatus disclosed in the patent may be used to remove fluorides, chlorides, cyanides, ammonia and sulfides from stack gas effluent.
U.S. Pat. No. 4,134,744 to Peterson et al discloses a fine particulate captive device. The apparatus disclosed therein captures particulate matter from a gas by the flow of a dielectric fluid across a rotating disc having charged segments for the attraction of the particulate matter. The particulate matter is captured in the dielectric fluid and then centrifuged into a means for collecting the fluid.
A principal objective of the Peterson et al patent is to provide an apparatus for capturing submicron size particulate matter from a gas, such as air, for analysis. The Peterson et al patent further suggests that such apparatus be operated with a filter for capturing larger particles, thereby indicating such apparatus may not be suitable for continuous analysis of such larger particles. Also, the capture of fine particulate in the dielectric fluid in Peterson et al relies upon electrostatic potentials directed to the rotating disc and not primarily upon impingement of such particles in the dielectric fluid.
A liquid scrubber for air sampling is described in the article "Simple Liquid Scrubber for Large-Volume Air Sampling", by Buchanan et al, appearing in Applied Microbiology, pages 1140-1144, Volume 23, No. 6, June 1972. The principle of operation of this scrubber is founded upon producing a fine mist in a rapidly moving airstream and collecting the airborne particles by such particles impinging in a film of liquid formed by the mist droplets impacting on the walls of the scrubber. In this scrubber, the collecting fluid is pumped via a needle into the throat of an inlet arm, whereby it is transformed into a fine mist by the airstream. The mist droplets are cast from the airstream onto the scrubber walls, thereby forming a continuous, helically moving liquid film. The airborne particles primarily are taken out of the airstream through impingement in this liquid film. Such liquid film is moved by the airstream to an outlet where fluid collection in a flask occurs assisted by applying a slight vacuum.
The Buchanan et al article on the liquid scrubber discussed in the previous paragraph indicates such scrubber is mainly applicable to the recovery of microorganisms from large volumes of air, or for use in work relating to airborne disease transmission. This article does not disclose or suggest whether such liquid scrubber could be used for sampling other forms of particulate or various particulate containing stack gases, such as Hall cell fume, for example. Regarding such other forms of particulate and stack gases, this article does not disclose whether such scrubber has any means for transporting an unbiased, representative sample of particulate species of interest from a stream source to such scrubber, if such scrubber could be so used for such particulate or if such scrubber could not be physically located in such stream.
In the paper "Advances in Large-Volume Air Sampling", by Dahlgren et al, apparently originating at the Chemical Systems Laboratory, Aberdeen Proving Ground, Md., various sampling devices are described for use in the collection of viable microorganisms from air. This paper discloses, inter alia, an air-to-air concentrator-wet collector having three stages. In the first two stages, selected particles of a specific size are concentrated into a lesser defined air volume. In the third stage, such concentrated sample volume is directed to a liquid scrubber, whereby the particles therein are removed from the airstream by impinging in a small amount of collecting fluid.
The Dahlgren et al paper of the previous paragraph describes the air-to-air concentrator-wet collector as being designed primarily for operation in ambient air for sampling viable microorganisms therefrom. This paper does not disclose whether such collector is suitable for other non-related sampling applications, such as sampling process streams comprised of various stack gases, for example. The Dahlgren et al paper does not indicate whether particulate plugging in any stage of such collector or in any sampling line bringing a particulate-containing stream to such collector would occur, if such collector were used for purposes not stated in this paper. As previously mentioned, particulate plugging in the sampling system or in any delivery line thereto may bias any analysis through the inability to collect a representative sample of the particulate.
What is needed, therefore, is a simple and efficient method and apparatus for continuous analysis of a gas and particulate stream from a variety of sources. Such method and apparatus should provide for collection of particulate species of interest to permit accurate and unbiased continuous analysis thereof.